Preparation of naphthalene carboxylic acids from alkyl-substituted tetrahydronaphthalenes



United States Patent Ofiice 3,247,247 Patented Apr. 19, 1956 Thisinvention relates to the production of oxidation products of aromaticcompounds, and more particularly relates to a novel method of preparingnaphthalene carboxylic acids by the oxidation of the correspondingalkylsubstituted tetrahydronaphthalenes.

The oxidation of tetrahydronaphthalene and its alkyl derivatives isknown in the prior art. Thus, for example, it is known to treattetrahydronaphthal'e'ne with oxygen. Also, tetrahydronaphthalene hasbeen oxidized with nitric acid. In each case, however, thea-carbon atomof the saturated ring has been attacked, thus resulting in theproduction of tetrahydronaphthalene hydroperoxide and its decompositionproducts, tetralone and tetralol. At times, ring rupture has alsooccurred, giving tetralic acid, phthalic acid and the like.

It has now been found in accordance with the present invention, thatalkyl tetrahydronaphthalenes may be converted to the correspondingnaphthalene carboxylic acids, with little or no conversion of thestarting material to its less desirable decomposition products, byoxidizing these alkyl tetrahydronaphthalenes with N0 gas at elevatedtemperatures in the presence of Se0 and a solvent which is substantiallyinert to N0 By oxidizing these alkyl tetrahydronaphthalenes in thismanner, it has been found that, most surprisingly and unexpectedly, asimultaneous dehydrogenation of the saturated ring together withoxidation of the alkyl groups to the corresponding carboxylic acid thusoccurs.

This invention may be practiced on relatively pure alkyltetrahydronaphthalene such as the monoalkyl, dialkyl, or trialkylcompounds, in which case the corresponding mono, di-, or tricarboxylicacid will be formed. Alkyl groups having from about l-6 carbon atoms aresatisfactorily employed, as for example methyl, ethyl, n-propyl,isopropyl, n-butyl groups or the like, but in each case, regardless ofthe length of the alkyl side chain, only the naphthalene carboxylic acidhaving a -COOH group attached directly to the ring nucleus is formed.Preferably, a dialkyl tetrahydronaphthalene such as 2,6-dimethyltetrahydronaphthalene, 1,5-dimethyl tetrahydronaphthalene or the like isemployed as the starting material, but other alkyltetrahydronaphthalenes as for example 2-ethyl tetrahydronaphthalene, or6-npropyl tetrahydronaphthalene may likewise be utilized in the practiceof this invention. In addition to practicing this invention withrelatively pure starting materials, the aforesaid compounds may also bemixed with alkyl naphthalenes, as for example 2,6-dimethylnaphthalene;1,6-dimethylnaphthalene; 1,3-diethylnaphthalene and like compounds,which when treated in accordance with the present process are alsoconverted to the corresponding naphthalene carboxylic acids. Thesemixtures of alkylnaphthalenes and alkyltetrahydronaphthalene may occurwhen hydrosulfurization is practiced on alkylnaphthalene concentrates,in which case some of the concentrate may be converted toalkyltetrahydronaphthalenes. Alkyltetrahydronaphthalenes may also beformed during the isomerization of mixed dimethyltetrahydronaphthalenes.

The process of this invention is conveniently carried out by dissolvingthe alkyltetrahydronaphthalene starting material in a solvent which isinert to N0 at elevated temperatures utilized in this reaction andbubbling N0 through the reaction mixture until the oxidation anddehydrogenation are complete. It has been found that various chlorinatedbenzenes are quite suit-able as solvents and preferably those havingfrom 1-4 chlorine atoms, particularly trichlorobenzene. However,depending upon the range of reaction conditions, other solvents such ashigher or lower chlorinated benzenes, nitrobenzenes, ethers such asdiphenyl-ether or chlorinated ethers and the like may also be employed.

A considerable range of temperatures may be used in this reaction. Theminimum temperature for operation is about C., and preferably it rangesfrom about -200 0, although higher temperatures up to the decompositionpoints of reactants and end products may be employed. The seleniumcatalyst may be added to the reaction mixture as the dioxide, or as HSeO dissolved in water; alternatively, it may be. prepared. in situ inthe reaction vesselby bubbling N0 gas through the solvent in thepresence of selenium metal at an elevated temperature, either prior tothe addition of the alkyl tetrahydronaphthalene or in small portions atequal intervals during the course of the reaction until the selenium isoxidized to the dioxide. The amount of selenium-needed to convert analkyl tetrahydronaphthalene to the corresponding naphthalene carboxylicacid generally ranges from 0.5 to 10 parts by weight of selenium toconvert 100 parts by weight of starting material. When a dimethyltetrahydronaphthalene is converted to naphthalene dicarboxylic acid, forexample, from two to six parts by weight of selenium to 100 parts byweight of dimethyl starting material is preferably employed.

In conducting this reaction, it is desirable that the amount of NOemployed be as stoichiometric as possible in order to insuresubstantially complete reduction of the N0 to NO, thus making therecovery of the NO and reconversion to NO a simple and completeoperation so that the N0 is recycled with practically no loss. Theregulation of the amount of N0 thus employed is readily achieved bymaking certain that the off-gases produced by the reaction areessentially colorless; since brown off-gases will be a clear indicationthat unconsumed N0 is passing through. Generally rates of about 1.5 to2.5 grams of N0 per minute are preferred. Alternatively, if necessary,when maximum yields in a onestage oxidation are desired, the reactionmay be continued for a period of time after brown off-gases are noted toinsure maximum oxidation and dehydrogenation of the starting material.

The naphthalene carboxylic acid reaction products are generallyinsoluble in the chlorobenzene solvent and may conveniently be recoveredby filtering and drying the precipitated solids. Thus, for example, inthe preparation of a naphthalene dicarboxylic acid, this product isreadily recovered by filtering the solids, washing them with a suitableorganic solvent such as hexane, heptane or the like, and drying thewashed solids in an oven. The acid may, if desired, then be furtherpurified by known crystallization methods, or alternatively,esteri'fied, as for example with a low molecular weight alcohol such asmethanol or ethanol at an elevated temperature to form the correspondingester or diester.

The following examples are given by way of illustration and are not tobe regarded as limitations of this invention:

Example I 1200 ml. of trichlorobenzene is heated to C. in a reactorfitted with a gas inlet tube, a side arm with a condenser, .athermometer, and a stirrer. One gram of selenium is added and N0 whichhas been preheated to 160 C. is bubbled through the flask at the rate ofabout dried in an oven.

tetrahydronaphthalene in 400 ml. of trichlorobenzene is' added in 25 ml.portions over a period of 4 hours. During this same period 3.75 grams ofadditional selenium is added to the reaction, the last grams of which isadded at. the end of the fourth hour. The reaction is then continued foran additional hour. The temperature of the reaction varies between190203 C., and 65 m1. of water is collected. The reaction mixture iscooled to room temperature, and the solids filtered, washed with hexane,and dried in an oven. The weight of the dry solids'is 171.5 grams. Thesolids have an acid number of 462 which corresponds to 76% of2,6-naphthalene dicarboxylic acid. r

In accordance with the foregoing procedure, but starting with2,7-'diethyl tetrahydronaphthalene, there is obtained the corresponding2,7-naphthalene dicarboxylic acid.

In accordance with the foregoing procedure, but starting with1,5-di-n-propyl tetrahydronaphthalene, there is obtained thecorresponding 1,5-naphthalene dicarboxylic acid.

Example 2 To 1000 ml. of trichlorobenzene preheated to 190 C. in areaction vessel identical to the one used in Example 1 is added 3 gramsof selenium. Nitrogen dioxide, preheated to 160 C. is bubbled throughthe flask at the rate of about 1.5 grams per minute. A solution of 100grams of a mixture of 2,6-dimethyl tetrahydronaphthalene and2,6-dimethy1 naphthalene in a ratio of about 1:9 by weight dissolved in500 ml. of trichlorobenzene is added in 25 ml. portions over a period of4 hours. During this same period 2 grams of additional selenium is addedto the reaction, the last gram of which is added at the end of the thirdhour. The temperature varies between 185-2O0 C. The'reaction mixture iscooled to room temperature, the solids filtered, washed with hexane andThe solids have an acid member of 478 which corresponds to 83% of2,6-naphthalene dicarboxylic acid.

In accordance with the foregoing procedure, but starting with a mixtureof 1,5-dimethyl tetrahydronaphthalene and 1,5-dimethyl naphthalene thereis recovered the corresponding 1,5-naphthalene dicarboxylic acid.

Example 3 To a reaction vessel fitted the same as in Example-1 is added1200 ml. of tn'chlorobenzene which is then heated rate of 2 gms./min. Asolution of grams of 6-methyl tetrahydronaphthalene in 400 ml. oftrichlorobenzene is added in 25 ml. of portions over a period of fourhours. During this same period 3.75 grams of additional selenium isadded to the reaction, the last gram of which is added at the end of thefourth hour. The reaction is then continued for an additional hour. Thetemperature of the reaction varies between 190-203 C., and 50 ml. ofwater is collected. The reaction mixture is cooled to room temperature,the solids filtered, washed with benzene, and dried in an oven to yieldZ-naphthoic acid.

In accordance with the foregoing procedure, the starting with 2-methyltetrahydronaphthalene, there is obtained the corresponding 2-naphthoicacid.

In accordance with the foregoing procedure, but starting with 5 n-propyltetrahydronaphthalene, there is obtained the corresponding lnaphthoicacid.

The invention claimed is:

1. The process which comprises reacting a lower alkyltetrahydronaphthalene with N0 in the presence of selenium dioxide and asolvent which is inert to N0 to produce the corresponding naphthalenecarboxylic acid, said reaction being conducted at a temperature in therange of from about 145 C. to the decomposition temperature of saidproducts.

2. The process according to claim 1 wherein the start ing material is adialkyl tetrahydronaphthalene.

3. The process according to claim 2 wherein the starting material is2,6-d-imethyl tetrahydronaphthalene.

4. The process according to claim 1 wherein the starting material is amixture of dialkyl tetrahydronaphthalenes and dialkyl naphthalenes.

5. The process according to claim 4 wherein the starting material is amixture of 2,6-dimethyl tetrahydronaphthalene and 2,6-dimethylnaphthalene.

6. The process according to claim 1 wherein the starting material is amonoalkyl tetrahydronaphthalene.

7. The process according to claim 1 wherein the starting material is2-methyl tetrahydronaphthalene.

8. The process according to claim 1 wherein the reaction is carried outat a temperature of from about C. to 200 C.

References Cited by the Examiner UNITED STATES PATENTS 2,428,590 10/1947 Shokal et a1. 252-439 X 2,860,162 11/ 1958 Ekenstam 260-524LORRAINE A. WEINBERGER, Primary Examiner. V DANIEL D. HORWITZ,.Examiner.

1. THE PROCESS WHICH COMPRISES REACTING A LOWER ALKYLTETRAHYDRONAPHTHALENE WITH NO2 IN THE PRESENCE OF SELENIUM DIOXIDE AND ASOLVENT WHICH IS INERT TO NO2 TO PRODUCE THE CORRESPONDING NAPHTHALENECARBOXYLIC ACID, SAID REACTION BEING CONDUCTED AT A TEMPERATURE IN THERANGE OF FROM ABOUT 145*C. TO THE DECOMPOSITION TEMPERATURE OF SAIDPRODUCTS.